Hello again, folks. A while back I asked about how to filter out a nearby 0.5 Hz signal from a 3-5 Hz signal. I mentioned that a high pass filter wouldn't work because it messes up the phase too much in the passband. The solution I ended up using was a High Q band-pass subtractive filter on the 0.5 Hz signal. One of the nice properties of this is that because the phase of the HQBP filter doesn't change by more than 90 deg, when you subtract it from the original signal, the frequency response looks like what you expect, e.g. a dip at a certain frequency, without any ripples or amplification at other frequencies due to the phase being off by too much. Also, because the attenuation is so low at 3 Hz, the phase is not messed up by much at all. For some reason, I didn't use a notch filter because it seemed more complicated.
When I tried using a low pass high order subtractive filter, the frequency responce was kind of weird because there are greater than 90 degree phase shifts. Anyway, I just posted this to let people know what worked for me.